82 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



and uniformly along the mid-line as at the sides. Practically all investigators, 

 who have studied the embryology of Amphioxus or the aseidians are in agreement 

 upon this point, and if concrescence occurs among the amphibians, as is claimed 

 by some investigators, though denied by others, it can only be said that in this 

 respect the amphibians are very different from these other classes. The evidence 

 that the amphibians do form such an exception is by no means conclusive, as Ziegler 

 (1902) points out, 



The question whether and to what extent there is an actual inrolling of cells 

 from the outer to the inner layer in the closure of the blastopore is one which has 

 been much discussed. In all three of these chordate classes an inrolling of cells at 

 the margin of the blastopore has been repeatedly observed, but the relative number, 

 the origin and the character of such cells are matters of dispute. Lwoff (1894) 

 maintains that the entire dorsal lip of Ampktoxus, inner as well as outer layer, is 

 formed from ectoderm cells which are inrolled. All of these inrolled cells he counts 

 as ectoderm and consequently concludes that the chorda and mesoderm are of ecto- 

 dermal origin. The invagination of the endoderm is, in his opinion, the real gastru- 

 lation. whereas the turning in of the ectodermal cells is a coenogenetic process 

 which has nothing to do with the formation of the enteron but is concerned only 

 with the formation of chorda and mesoderm. This conclusion has been criticised 

 by Samassa (1898). Klaatsch (1896), Morgan and Hazen (1900). et al., on the 

 ground that there is no sufficient evidence that the inrolled cells are ectodermal. 

 With this conclusion, when extended to the aseidians. I heartily agree. Here 

 the cells which are inrolled at the anterior border of the blastopore are chorda 

 cells which are yolk laden and resemble endoderm and not ectoderm. The cells 

 which are inrolled at the posterior lateral borders are mesenchyme and muscle cells 

 and in histological structure are very unlike the ectoderm. While therefore agree- 

 ing with Lwoff that the chorda and mesoderm cells are inrolled (though from 

 opposite portions of the blastopore lip in aseidians) I agree with his critics that 

 these cells, judged by their lineage and histological character, are certainly not 

 ectodermal. 



G. Neural Plate. 



In aseidians the neural plate material becomes segregated into six cells at the 

 44-cell stage ; four of these cells lie in a transverse row at the anterior border of 

 the dorsal hemisphere, just below the third cleavage plane and two of them lie 

 just above this plane and therefore in the ventral hemisphere. The four dorsal 

 cells lie just anterior to the four chorda cells from which they were separated at the 

 sixth cleavage. Both- the dorsal and ventral cells divide transversely, the former 

 sjivimz; rise to an arc of eight cells the latter to one of four cells, and to these a 

 single additional cell is added on each side making an arc of six neural plate cells 

 in the ventral hemisphere. In subsequent divisions the neural plate increases much 

 in length and its anterior portion also increases in breadth, but it is never more than 

 eight cells wide in its posterior part, Soon after gastrulation begins the neural 

 cells overgrow the chorda cells and thereafter cover the dorsal lip to its posterior 



